Twenty-four male students were selected, from an initial pool of seventy-five, to adopt randomly assigned roles of prisoner and guard, in a mock prison, situated in the basement of the Stanford Psychology Department building, for a period of between seven and fourteen days. The participants adapted to their roles well beyond Zimbardo's expectations, as the guards enforced authoritarian measures and ultimately subjected some of the prisoners to psychological torture. Many of the prisoners passively accepted psychological abuse and, at the request of the guards, readily harassed other prisoners who attempted to prevent it. The experiment even affected Zimbardo himself, who, in his role as the superintendent, permitted the abuse to continue.[5][6] Two of the prisoners quit the experiment early, and the entire experiment was abruptly stopped after only six days, to an extent because of the objections of Christina Maslach. Certain portions of the experiment were filmed, and excerpts of footage are publicly available.

Zimbardo and his team aimed to test the hypothesis that the inherent personality traits of prisoners and guards are the chief cause of abusive behavior in prison. Participants were recruited and told they would participate in a two-week prison simulation. Out of 75 respondents, Zimbardo and his team selected the 24 males whom they deemed to be the most psychologically stable and healthy.[7] These participants were predominantly middle-class.[8] The group was intentionally selected to exclude those with criminal background, psychological impairments or medical problems. They all agreed to participate in a 7–14-day period and received $15 per day (equivalent to $87 in 2015).

The experiment was conducted in the basement of Jordan Hall (Stanford's psychology building). Twelve of the twenty-four participants were assigned the role of prisoner (nine plus three alternates), while the other twelve were assigned the role of guard (also nine plus three alternates). Zimbardo took on the role of the superintendent, and an undergraduate research assistant the role of the warden. Zimbardo designed the experiment in order to induce disorientation, depersonalization and deindividualization in the participants.

The researchers held an orientation session for guards the day before the experiment, during which they instructed them not to physically harm the prisoners. In the footage of the study, Zimbardo can be seen talking to the guards: "You can create in the prisoners feelings of boredom, a sense of fear to some degree, you can create a notion of arbitrariness that their life is totally controlled by us, by the system, you, me, and they'll have no privacy ... We're going to take away their individuality in various ways. In general what all this leads to is a sense of powerlessness. That is, in this situation we'll have all the power and they'll have none."[9]

The researchers provided the guards with wooden batons to establish their status,[10] clothing similar to that of an actual prison guard (khaki shirt and pants from a local military surplus store), and mirrored sunglasses to prevent eye contact. Prisoners wore uncomfortable ill-fitting smocks and stocking caps, as well as a chain around one ankle. Guards were instructed to call prisoners by their assigned numbers, sewn on their uniforms, instead of by name.

The prisoners were "arrested" at their homes and "charged" with armed robbery. The local Palo Alto police department assisted Zimbardo with the arrests and conducted full booking procedures on the prisoners, which included fingerprinting and taking mug shots. They were transported to the mock prison from the police station, where they were strip searched and given their new identities.

The small mock prison cells were set up to hold three prisoners each. There was a small space for the prison yard, solitary confinement, and a bigger room across from the prisoners for the guards and warden. The prisoners were to stay in their cells all day and night until the end of the study. The guards worked in teams of three for eight-hour shifts. The guards did not have to stay on site after their shift.

After a relatively uneventful first day, on the second day the prisoners in Cell 1 blockaded their cell door with their beds and took off their stocking caps, refusing to come out or follow the guards' instructions. Guards from other shifts volunteered to work extra hours to assist in subduing the revolt, and subsequently attacked the prisoners with fire extinguishers without being supervised by the research staff. Finding that handling nine cell mates with only three guards per shift was challenging, one of the guards suggested that they use psychological tactics to control them. They set up a "privilege cell" in which prisoners who were not involved in the riot were treated with special rewards, such as higher quality meals. The "privileged" inmates chose not to eat the meal in order to stay uniform with their fellow prisoners. After only 36 hours, one prisoner began to act "crazy", as Zimbardo described: "#8612 then began to act crazy, to scream, to curse, to go into a rage that seemed out of control. It took quite a while before we became convinced that he was really suffering and that we had to release him."

Guards forced the prisoners to repeat their assigned numbers[11] to reinforce the idea that this was their new identity. Guards soon used these prisoner counts to harass the prisoners, using physical punishment such as protracted exercise for errors in the prisoner count. Sanitary conditions declined rapidly, exacerbated by the guards' refusal to allow some prisoners to urinate or defecate anywhere but in a bucket placed in their cell. As punishment, the guards would not let the prisoners empty the sanitation bucket. Mattresses were a valued item in the prison, so the guards would punish prisoners by removing their mattresses, leaving them to sleep on concrete. Some prisoners were forced to be naked as a method of degradation. Several guards became increasingly cruel as the experiment continued; experimenters reported that approximately one-third of the guards exhibited genuine sadistic tendencies. Most of the guards were upset when the experiment concluded after only six days.

Zimbardo mentions his own absorption in the experiment. On the fourth day, some of the guards stated that they heard a rumor that the released prisoner was going to come back with his friends and free the remaining inmates. Zimbardo and the guards disassembled the prison and moved it onto a different floor of the building. Zimbardo himself waited in the basement, in case the released prisoner showed up, and planned to tell him that the experiment had been terminated. The released prisoner never returned, and the prison was rebuilt in the basement once again.

Zimbardo argued that the prisoners had internalized their roles, since some had stated that they would accept "parole" even if it would mean forfeiting their pay, despite the fact that quitting would have achieved the same result without the delay involved in waiting for their parole requests to be granted or denied.[12] Zimbardo argued they had no reason for continued participation in the experiment after having lost all monetary compensation, yet they did, because they had internalized the prisoner identity.

Prisoner No. 416, a newly admitted stand-by prisoner, expressed concern over the treatment of the other prisoners. The guards responded with more abuse. When he refused to eat his sausages, saying he was on a hunger strike, guards confined him to "solitary confinement", a dark closet: "the guards then instructed the other prisoners to repeatedly punch on the door while shouting at 416."[13] The guards stated that he would be released from solitary confinement only if the prisoners gave up their blankets and slept on their bare mattresses, which all but one refused to do.

Zimbardo aborted the experiment early when Christina Maslach, a graduate student in psychology whom he was dating (and later married),[14] objected to the conditions of the prison after she was introduced to the experiment to conduct interviews. Zimbardo noted that, of more than fifty people who had observed the experiment, Maslach was the only one who questioned its morality. After only six days of a planned two weeks' duration, the Stanford prison experiment was discontinued.[12]

On August 20, 1971, Zimbardo announced the end of the experiment to the participants. The results of the experiment have been argued to demonstrate the impressionability and obedience of people when provided with a legitimizing ideology and social and institutional support. The experiment has also been used to illustrate cognitive dissonance theory and the power of authority.

The results of the experiment favor situational attribution of behavior rather than dispositional attribution (a result caused by internal characteristics). In other words, it seemed that the situation, rather than their individual personalities, caused the participants' behavior. Under this interpretation, the results are compatible with the results of the Milgram experiment, in which ordinary people fulfilled orders to administer what appeared to be agonizing and dangerous electric shocks to a confederate of the experimenter.[citation needed]

The guards and prisoners adapted to their roles more than Zimbardo claimed to have expected, stepping beyond predicted boundaries, leading to dangerous and psychologically damaging situations. One-third of the guards were judged to have exhibited "genuine sadistic tendencies", while many prisoners were emotionally traumatized, as five of them had to be removed from the experiment early. After Maslach confronted Zimbardo and forced him to realize that he had been passively allowing unethical acts to be performed under his supervision, Zimbardo concluded that both prisoners and guards had become grossly absorbed in their roles and realized that he had likewise become as grossly absorbed in his own, and he terminated the experiment.[15] Ethical concerns surrounding the experiment often draw comparisons to a similar experiment, which was conducted ten years earlier in 1961 at Yale University by Stanley Milgram.[citation needed]

Because of the nature and questionable ethics of the experiment, Zimbardo found it impossible to keep traditional scientific controls in place. He was unable to remain a neutral observer, since he influenced the direction of the experiment as the prison's superintendent. Conclusions and observations drawn by the experimenters were largely subjective and anecdotal, and the experiment is practically impossible for other researchers to accurately reproduce. Erich Fromm claimed to see generalizations in the experiment's results, and argued that the personality of an individual does affect behavior when imprisoned. This ran counter to the study's conclusion that the prison situation itself controls the individual's behavior. Fromm also argued that the amount of sadism in the "normal" subjects could not be determined with the methods employed to screen them.[16]

"John Wayne" (the real-life Dave Eshelman), one of the guards in the experiment, said that the study placed undue emphasis on the cruelty of the guards, and that he caused the escalation of events between guards and prisoners after he began to emulate a character from the Paul Newman film Cool Hand Luke. He further intensified his actions because he was nicknamed "John Wayne," even though he was trying to mimic actor Strother Martin, who had played the role of the sadistic Captain in the movie.[17]

Also, researchers from Western Kentucky University argued that selection bias may have played a role in the results. The researchers recruited students for a study using an advertisement similar to the one used in the Stanford Prison Experiment, with some ads saying "a psychological study" (the control group), and some with the words "prison life" as originally worded in Dr. Zimbardo's Stanford Prison Experiment. It was found that students who responded to the classified advertisement for the "prison study" were higher in traits such as social dominance, aggression, authoritarianism, etc. and were lower in traits related to empathy and altruism when statistically compared to the control group participants.[18]

The study has been criticized for demand characteristics by psychologist Peter Gray. He argues that participants in psychological experiments are more likely to do what they believe the researchers want them to do. The guards were essentially told to be cruel. However, it was precisely this willingness to comply with the experiment's questionable practices that showed how little was needed for the students to engage in such practices.[19]

Most of the Stanford guards did not exhibit any cruel or unusual behavior, often being friendly and doing favors for the prisoners...The statistical validity of the sample of participants, 24 male Stanford students of about the same age, has been called into question as being too small and restrictive to be generally applicable to the population at large...(and the fact that) Zimbardo has dedicated much of his career to the promotion of the idea that bad environments drive bad behavior.[20]

Criticism of the scientific validity and ethics Stanford prison experiment has come from the participants themselves [21]. John Mark, a guard, talked of how the dramatic tension was staged:

I didn't think it was ever meant to go the full two weeks. I think Zimbardo wanted to create a dramatic crescendo, and then end it as quickly as possible. I felt that throughout the experiment, he knew what he wanted and then tried to shape the experiment—by how it was constructed, and how it played out—to fit the conclusion that he had already worked out. He wanted to be able to say that college students, people from middle-class backgrounds—people will turn on each other just because they're given a role and given power. Based on my experience, and what I saw and what I felt, I think that was a real stretch. I don't think the actual events match up with the bold headline. I never did, and I haven't changed my opinion. - John Mark, The Menace Within, Stanford Alumni Magazine

David Eshelman, described as one of the most sadistic guards, echoed this impression of dramatic staging and claimed that the researchers' intention to demonstrate the cruelty of prison guards was clear, and something the guards played up to:

What came over me was not an accident. It was planned. I set out with a definite plan in mind, to try to force the action, force something to happen, so that the researchers would have something to work with. After all, what could they possibly learn from guys sitting around like it was a country club? So I consciously created this persona. I was in all kinds of drama productions in high school and college. It was something I was very familiar with: to take on another personality before you step out on the stage. I was kind of running my own experiment in there, by saying, "How far can I push these things and how much abuse will these people take before they say, 'knock it off?'" But the other guards didn't stop me. They seemed to join in. They were taking my lead. Not a single guard said, "I don't think we should do this." - David Eshelman, The Menace Within, Stanford Alumni Magazine

Lynndie England pointing to a naked prisoner being forced to masturbate in front of his captors[22]

When acts of prisoner torture and abuse at the Abu Ghraib prison in Iraq were publicized in March 2004, Zimbardo himself, who paid close attention to the details of the story, was struck by the similarity with his own experiment. He was dismayed by official military and government representatives' shifting the blame for the torture and abuses in the Abu Ghraib American military prison on to "a few bad apples" rather than acknowledging it as possibly systemic problems of a formally established military incarceration system.

Eventually, Zimbardo became involved with the defense team of lawyers representing one of the Abu Ghraib prison guards, Staff Sergeant Ivan "Chip" Frederick. He was granted full access to all investigation and background reports, and testified as an expert witness in SSG Frederick's court martial, which resulted in an eight-year prison sentence for Frederick in October 2004.

Zimbardo drew from his participation in the Frederick case to write the book The Lucifer Effect: Understanding How Good People Turn Evil, published by Random House in 2007, which deals with the striking similarities between his own Stanford Prison Experiment and the Abu Ghraib abuses.[13]

While Haslam and Reicher's procedure was not a direct replication of Zimbardo's, their study casts further doubt on the generality of his conclusions. Specifically, it questions the notion that people slip mindlessly into role and the idea that the dynamics of evil are in any way banal. Their research also points to the importance of leadership in the emergence of tyranny of the form displayed by Zimbardo when briefing guards in the Stanford experiment.[24][25]

The Third Wave was a 1967 recreation of Nazi Party dynamics by high school teacher Ron Jones in Palo Alto, California. Although the veracity of Jones' accounts has been questioned,[26] several participants in the study have gone on record to confirm the events.[27]

In 1977, Italian director Carlo Tuzii adapted the experiment to an Italian environment. Italian students made a film based on it, La Gabbia (The Cage).

In 1992, Quiet Rage: The Stanford Prison Experiment, a documentary about the experiment, was made available via the Stanford Prison Experiment website. The documentary was written by Zimbardo and directed and produced by Ken Musen.[28]

In 2010, Inferno Distribution released the film The Experiment, which is an English-language remake of the 2001 film Das Experiment.

In an October 2008 episode of the NBC television show Life, Detectives Crews and Reese investigated a murder that took place at a prison experiment loosely modeled on the Stanford Prison Experiment.

In the third season of the television series Veronica Mars, a variant of the experiment is recreated as an activity for a sociology class, the main difference being that the guards were expected to get information out of the prisoners.

Zimbardo, P. G. (1971). "The power and pathology of imprisonment", Congressional Record (Serial No. 15, 1971-10-25). Hearings before Subcommittee No. 3, of the United States House Committee on the Judiciary, Ninety-Second Congress, First Session on Corrections, Part II, Prisons, Prison Reform and Prisoner's Rights: California. Washington, DC: U.S. Government Printing Office.